1. Field of the Invention
The present invention relates to a method for deinterlacing video signals, and more particularly, to an apparatus and method for deinterlacing video signals, in which clearer picture quality can be secured by solving a variety of phenomena, such as step pattern image, horizontal moire pattern afterimage, noise and the like, which occur when an interlaced scanning image consisting of fields is converted into a progressive scanning image consisting of the same number of frames according to image modes. In addition, a still image, a film mode image and a caption image can be displayed in a resolution close to a photograph and also restored closely to an original image, thereby improving a picture quality.
2. Description of the Related Art
Generally, a display of interlaced scanning images is implemented by alternating odd fields (i.e., an odd line group of screen) and even fields (i.e., an even line group of screen) with a predetermined time difference.
An interlaced scanning system includes a typical analog television, a video camera, a video cassette recorder (VCR) and the like. The system's signal processing, such as transmission, storage and display, is performed in the interlaced scanning method. In digital televisions of ATSC standard, 1920×1080i and 720×480i formats are based on the interlaced scanning method. A progressive scanning system is focused on personal computers (PCs) and a display of most PCs is achieved using the progressive scanning method. In digital televisions, 1920×1080p, 1280×720p and 720×480p formats are based on the progressive scanning method. The reason why the image processing is classified into two methods, i.e., the interlaced scanning method and the progressive scanning method is that contents of to-be-displayed images are different from each other. In other words, in case the same frequency bandwidth is given, the interlaced scanning method is advantageous to the display of moving images because it can provide an excellent picture quality. Meanwhile, the progressive scanning method is advantageous to the display of PC images having a large number of lines, dots and still images.
However, in case the moving images and the PC images are simultaneously displayed on one device, it is usual to employ the progressive scanning method. The reason is that degradation of the picture quality is conspicuous when the still images consisting of dots and lines are displayed based on the interlaced scanning method. In addition, the scanning method is different according to the display devices. A display device employing CRT that is widely used in TVs must perform a high voltage deflection. Therefore, the interlaced scanning method using a low horizontal frequency is advantageous to slightly vary a high voltage current. On the other hand, a flat panel display device employing PDP, LCD and DLP does not perform the high voltage deflection. Accordingly, the flat panel display device uses the progressive scanning method rather than the interlaced scanning method, since the interlaced scanning method has drawbacks of line flicker, large area flicker, detection of a scanning line, and the like.
In the digital era, most images are processed in a digital mode. The digitally processed images may be stored in HDD of PC or displayed on PC screen. Meanwhile, there are many users who want to display PC images on TV screens. With the advance of large-sized screen/high definition in a digital TV era, the focus gradually moves from the CRT based display to the flat panel display, since the flat panel display is light in weight and can provide a large-sized screen.
For these reasons, a deinterlace process is required which converts the interlaced scanning moving images into the progressive scanning moving image. A line repetition method, an intra-field interpolation method, an inter-field interpolation method and a motion adaptive interpolation method are proposed for implementing the deinterlace process with simple hardware.
According to the line repetition method, a line to be interpolated is generated by repeating a line disposed above in the same field. Although the line repetition method can be implemented with the simplest hardware, it has a disadvantage that degrades the picture quality since a boundary of an inclined line seems a stepped shape after the interpolation.
According to the intra-field interpolation method, pixels disposed below and above in the same field as a pixel to be interpolated are used to obtain pixel value to be interpolated through an arithmetical calculation such as addition and division. The intra-field interpolation method can reduce the step phenomenon much more than the line repetition method. However, the intra-field interpolation method makes frames by using only information on one field with respect to the still image, thus lowering a vertical resolution by half.
The inter-field interpolation method is performed by taking and inserting the line disposed at the same position of a just previous field into the to-be-interpolated line of a current field. The inter-field interpolation method can obtain an excellent vertical resolution with respect to the still image. However, since this is similar to the overlap of two images, whose timings are somewhat different from each other, the inter-field interpolation method has a disadvantage in that the flicker may occur in a screen when the moving image is entirely displayed. Further, the horizontal moire pattern afterimages may occur in a motion direction of objects in a screen when the moving image is locally displayed.
According to the motion adaptive interpolation method, motion states of adjacent pixels are detected at a position of a pixel to be interpolated, and either of the intra-field interpolation method and the inter-field interpolation method is selected according to the detected motion value. Although the motion adaptive interpolation method can obtain an excellent vertical resolution with respect to the moving image and the still image compared with other methods, an interpolation function for various types of images is degraded. Particularly, a partial flicker occurs seriously in an image having much noise. In case most of the images are the moving images and some of them are the still images, noise occurs at a boundary area between the moving image and the still image due to noise occurring in the extraction of the motion value. Since the motion values are generated among odd fields and among even field, it is impossible to extract a motion in a very fast image, thus occurring a horizontal moire pattern afterimage.
It is desirable not to use the above-described hardware because the conversion of the interlaced scanning image into the progressive scanning image results in the degradation of the picture quality. Although hardware is complicated, one method for solving the above problems is to detect the motion states of pixels disposed adjacent to the to-be-interpolated pixel and then select one of the intra-field interpolation method and the inter-field interpolation method according to the detected motion value. In this case, the inter-field interpolation method is performed with respect to the still object so as to maintain the vertical resolution, and the intra-field interpolation method is performed with respect to the moving object so as to prevent the horizontal moire pattern afterimage. Such a deinterlace method is called an improved motion adaptive deinterlace method. According to the improved motion adaptive deinterlace method, complexity and improvement in the picture quality are greatly changed according to the number of field memory devices used to detect the motion. Generally, the improved motion adaptive deinterlace method is implemented with a history in order to make it possible to provide 32 fields by using two or three field memory devices.